Disable generic numeric literals

[bishabosha]

Generic numeric literals should be under some sort of flag by 3.0 release.

Additionally, the scala.util.FromDigits classes should probably be moved to some experimental package or external library

Minimized code

this can be done in 3.0.0-M1 nightlies

case class FooNum(toInt: Int)

given util.FromDigits[FooNum]:
  def fromDigits(digits: String): FooNum = FooNum(BigInt(digits).toInt)

@main def Test =
  val foo: FooNum = 23
  println(foo)

Output

FooNum(23)

Expectation

should not compile without a flag

[odersky]

Fixed by #10286